1. Field of the Invention
The present invention relates to a high-frequency radio equipment system suitable for use in the Near Field Communication method and particularly with a control method for a high-frequency radio equipment and a high-frequency radio equipment system which lowers code error rate in a communication environment and realizes favorable communication.
2. Description of the Related Art
The “Near Field Communication method” uses a carrier band signal of an extremely low power which is not regulated by laws and regulations, for example.
As an example of a radio equipment system using the Near Field Communication method, a keyless entry control system is known. This is a system for controlling opening/closing of a vehicle door, start or stop of an engine, etc. to be controlled by radio from a position several to several tens of meters away and it is constituted by a fixed-type base station device mounted on the vehicle side and a portable (mobile) equipment carried by a user in a one-to-one relation.
According to the Enforcement Regulation of Radio Law, the field strength at a point 3 m away from a transmitter in the case of the frequency of not more than 322 MHz or not less than 150 GHz, as shown in FIG. 10, is not regulated irrespective of methods or application if it is 500 per meter [μV/m] or less. The other frequencies vary in the stepwise manner as shown in FIG. 10.
FIG. 10 is a diagram showing field strength values vs. frequencies not regulated by Radio Law.
In the past, the one-to-one Near Field Communication method in an extremely low power not regulated by the Enforcement Regulation of Radio Law was general less affected by multipath fading, interference, etc.
That is because, while mutual interference is likely to occur in the frequency band where many communication systems are mixed such as the Industrial Scientific Medical (ISM) band (2.4 GHz band) used for low-power data communication systems including a wireless Local Area Network (LAN) and with a rather high transmission power, the frequency used in many systems is as low as 322 MHz or less in the communication in an extremely low power and its wavelength is long and communication distance is short due to low transmission power, and such a problem hardly occurs.
Next, a conventional radio equipment system will be described referring to FIG. 11. FIG. 11 is a schematic block diagram showing a conventional radio equipment system. The system in FIG. 11 assumes a case applied to a keyless entry control system.
In the conventional radio equipment system, as shown in FIG. 11, a transmitter 61, which is a radio equipment (portable equipment) carried by a user modulates a carrier band signal at a frequency of the VHF band or above with an identification code of the transmitter 61 and an indication signal indicating opening/closing of a vehicle door, for example, and transmits it to a receiver 63, which is a fixed-type radio equipment (base station) through an antenna 62.
The receiver 63 has a filter for filtering the received carrier band signal and removing an unnecessary frequency component, and in the keyless entry control system, a high frequency of 315 MHz is used as frequency in the bandwidth required for communication.
Therefore, a Surface Acoustic Wave (SAW) filter suitable for application at a high frequency is generally used for the filter of the receiver 63.
Also, according to the Enforcement Regulation of Radio Law, the field strength at a point 3 m away from the transmitter 61 is not regulated if it is 500 per meter [μV/m] or less, as shown in FIG. 10.
The receiver 63 receives the carrier band signal transmitted in such an extremely low power at an antenna 64, filters it with a SAW filter 65, amplifies it at a frequency amplifier part (preamp) 66 to a predetermined power, performs direct detection of the amplified carrier band signal with a receiving mechanism 67 and demodulates the identification code and the indication signal.
And various driving mechanisms of the vehicle are controlled according to the indication signal, and the door is opened/closed.
As a prior art relating to the keyless entry control system, there is the JP-A-2000-27504 “Remote Keyless Entry System” (Applicant: Hitachi Ltd.; Inventor: Takashi Komata) published on Jan. 25, 2000.
This prior art describes that in a keyless entry control system, means for generating a plurality of transmission data rates are provided on an actuation device (portable equipment) and an operation device (base station) in the bidirectional communication, transmission is made at a high speed data rate, first, and if there is no response, transmission is made while sequentially lowering the data rate.
Also, as a prior art for changing a data transmission rate while performing directive control by an adaptive array antenna, there is JP-A-2002-076985 “Base Station Device, Communication Terminal Device, and Communication Method” (Applicant: Matsushita Electric Industrial Co., Ltd.; Inventor: Takahisa Aoyama et al.) published on Mar. 15, 2002.
In this prior art, if a modulation means is in a favorable state of transmission environment, high-speed download data is modulated in the high-speed rate modulation method at a base station for directive control with an adaptive array antenna.
This prior art, however, performs directive control by weighting a signal received by the adaptive array antenna, and not by adjusting receiving sensitivity by directly selecting a plurality of receiving antennas using a code error rate. Moreover, a transmission rate of a signal to be sent from a communication terminal device is not made variable.
Also, as a prior art for selecting a different chip rate according to a mobile communication device, there is the JP-A-2000-307478 “Device, System, and Method for Mobile Communication” (Applicant: Matsushita Electric Industrial Co., Ltd.; Inventor: Tadashi Hayakawa) published on Nov. 2, 2000.
In this prior art, a mobile communication device transmits an information transmission symbol rate, a target value of communication quality and a chip clock to a base station, the base station determines availability and responds to the mobile communication device, and if communication is available, communication is made based on it.
Therefore, in this prior art, an inquiry from the mobile communication device under a specific condition is required at first and the communication condition is determined by the inquiry and response, and this is not to make transmission rate variable while controlling directivity of an antenna from the code error rate.
However, the conventional radio equipment systems have a problem that, if there are many equivalent systems in the extreme vicinity in the area or in a radio wave environment where there is more than a little steady interference from other systems, code error rate increases, thereby lowering receiving sensitivity or deteriorating transmission quality.